(a) Field of the Invention
This invention relates to an epidark illumination system wherein the light from a light source is made to pass through a space formed between a sleeve and an objective lens to illuminate a sample.
(b) Description of the Prior Art
Generally, an epidark illumination system is of such formation as is shown in FIG. 1 wherein the light from a light source 1 will be passed through a stop 2 having a ring-shaped aperture and located in front of the light source, will be once made a parallel light pencil by a condenser lens 3, will then be reflected by a ring-shaped mirror 4 coaxial with the optical axis of an observing optical system and arranged obliquely, will then be passed through a cylindrical space formed by an objective lens 5 and a sleeve 6 provided around it and will be reflected by a ring-shaped conical mirror 7 provided in front of and coaxially with the objective lens 5 to illuminate a sample 8. In such epidark illumination system, the illuminating light pencil is formed cylindrically, the light pencil is cut by bridges for holding the objective lens 5 within the sleeve 6, the incident angle of the illumination light must be so selected that the light directly reflected from the sample 8 does not enter the objective lens 5 and therefore it is difficult to make such uniform illumination as the Kohler illumination utilized in a transmitted illumination system. Therefore, various attempts have been made to eliminate the uneven light amount (ununiform illumination) of the illuminating system. As one of them, a frosted glass-like optical member is placed within the cylindrical space between the sleeve and objective lens to thereby diffuse the illuminating light pencil and to reduce the ununiform illumination. Further, even in an objective lens of a high magnification, in order to lead the light diffused by a diffusing member onto the sample lest the directly reflected light from the sample surface should enter the objective lens, a light collecting lens is provided near the front end of the objective lens or the reflecting mirror is made to have a curvature. However, in such method as in the above, the loss of the light amount is large due to the diffusion of the light by the diffusing member. If such loss of the light amount is to be reduced, the uneveness of illumination will not be able to be well corrected.